Integration of a number of populated printed circuit boards originating from various designers into a single system is simplified by using a backplane standard, such as a VMEbus. The backplane may be fixed to a card rack having slots for insertion of the printed circuit boards. Connectors on the backplane are directed to receive the boards to allow electrical communication among the boards. Under the present VMEbus specification, a backplane may be sufficiently wide to interconnect 21 daughter boards.
While a particular backplane standard has a specific pin assignment for power, ground and signals, there is typically a great amount of flexibility in the bus specification for the standard. For example, the VMEbus specification allows a VMEbus system to be based upon a J1 backplane or upon a J1 and J2 backplane. Provided that the utility pin assignment dictated by the bus specification is met, a backplane manufacturer may have its own design for power distribution across the backplane. One aspect of performance of a backplane relates to the ability of the backplane to distribute power within specified voltage tolerances to each daughter board in a bus system.
Another aspect of performance is the ability of a backplane to convey signals among the daughter boards at the operating speed of the system in a manner in which each signal is understood at a receiving end. Multiprocessing capability provides a number of advantages, but leads to complexity that requires exacting design engineering to maximize the capability.
Factors that affect signal performance and that are within the control of a backplane manufacturer include the type and quality of substrate material, the amount of copper in a ground system, the placement of ground planes relative to signal planes, the uniformity of signal trace widths across the length of the signal traces, and the location of terminations. Even within a particular backplane standard, performance-effecting structural differences exist between the products of one backplane manufacturer and those of another. Depending upon quality control tolerances of a particular manufacturer, signal performance may even vary significantly among backplanes produced from a single source.
Thus, it would be to the benefit of users to have availability to diagnostic tools for characterization of a backplane. "Characterization" is defined as testing of a product in order to determine the limits of operation of the product. The VMEbus specification requires that signals pass across a backplane having 21 populated slots at a clock frequency of at least 16 MHz. However, a user may desire operation at 32 MHz. Characterization testing would allow a user to determine the upper limit of operational speed. Likewise, characterization of ground shift from a board in one slot to a board in a second slot may be important to ensuring that an acceptable signal-to-noise ratio will be achieved by utilizing a particular backplane.
For many users of backplanes, characterization testing to determine limits of operation is not the primary concern. Rather, validation is the focus. "Validation" is defined as testing to confirm proper operation of a given configuration of a system. There are a number of factors outside the control of a backplane manufacturer that influence signal performance and power performance. Validation assures a user that a fully integrated system will operate properly.
It is an object of the present invention to provide a diagnostic device and system for validation and characterization of a bus system that includes a backplane.